Photographs are increasingly produced by electronic methods of image capture and are stored in their original format, compressed or otherwise. It is in this form that they are preserved in general, on storage media such as computer hard disks, or recordable digital optical disks (CD ROM, DVD, etc.), or else flash memories. Currently most of the general public's digital photographs are stored in this way.
The question of the permanence of these storage means arises, firstly with regard to the lifetime of these media; a hard disk has an estimated lifetime of a few years with the continual risk of a total failure; recordable optical storage media have estimated lifetimes of several tens of years when not in use; during use, the appearance of scratches can dramatically reduce the lifetime. But the question also arises with regard to the permanence of the coding formats which may become obsolete over long storage durations and notably when concerned with the archiving of images.
U.S. Pat. No. 6,442,296 has already proposed that the raw image be etched directly onto a medium of microfilm type or an optical disk. The raw image is etched in a very reduced space by direct writing with a laser, each image pixel is represented by engraved or non-engraved points. If the image is coloured several images are stored, corresponding to the decomposition of the initial image into three primary colours. If the image contains grey levels, the pixels are downscaled by screening: a pixel is represented by several engraved or non-engraved points. The images can be recovered by a reading method not relying on the use of a decoder and they can also be observed directly by microscope.
Patent EP1310950 describes the same principle of direct archiving of images on an optical disk. The images are engraved by near-field writing optics and can be observed directly by microscope.